This invention relates to ballasts for ultraviolet (UV) or other gas discharge lamps that include an indicator of the on or off condition of the lamp. More particularly, the invention relates to ballasts including a pair of nodes that have voltage across them when a lamp operates normally, and that have substantially no voltage across them when the lamp is off. This condition typically occurs in ballasts that power lamps having heated filaments.
Ultraviolet (UV) lamps are widely used for sterilization of water and air in water supplies, air ventilation systems and the like. UV lamps are quite similar to conventional low pressure discharge lamps, the principal difference being that there is no fluorescent coating on the inside of the lamp. Without a fluorescent coating, UV radiation from the plasma inside the lamp flows directly out of the lamp, but is almost imperceptible to the human eye. In particular, the human eye is very insensitive to UV radiation, and may perceive only a faint, dim glow even when the intensity is such that immediate damage to the eye and other parts of the body may occur. Consequently this kind of UV light is dangerous to people, and UV light sources are always kept completely enclosed, for instance, inside a water tank or air duct where they sterilize the water or air.
Faced with the foregoing safety concern, the otherwise routine issue of verifying that the lamp is still running becomes a somewhat complex problem. Verification is important because failure of a UV lamp to sterilize the air or water may have serious health consequences. It is not acceptable for a person to view the light source to verify its operation. Instead, it is customary to provide some kind of electrical sensing circuitry to indicate satisfactory operation of the lamp.
One prior art approach involves modifying a lamp ballast to include a resistor in series with a UV lamp. The voltage across the resistor is used to drive a bidirectional light-emitting diode (LED). When lamp current is flowing, a voltage generated across the resistor is used to drive the bidirectional LED. A problem with this technique is that it only works for instant-start ballasts, which have only one wire going to the lamp ends. Also, it has the property that the UV ballast has to be constructed quite differently from a conventional ballast for gas discharge lamps such as fluorescent lamps. For instance, the mentioned resistor needs to be interconnected within the ballast circuitry, and wires for the LED need to be connected from the ballast circuitry to a point outside the ballast.
Another technique to provide an indication of on-off UV lamp operation is to place a light pipe close to the UV lamp to receive UV energy, and then to place a fluorescent element at an external end of the light pipe that lights up from the UV energy impinging upon it. The use of light pipes with fluorescent elements is relatively expensive and undesirable for that reason.
It would be desirable to provide an on-off indicator signal for a UV lamp.
If would be further desirable to provide an on-off indicator for a UV lamp that may incorporate a mass-produced ballast used to power fluorescent lamps.
If would also be desirable to provide an on-off indicator signal for non-UV lamps that may be economically implemented.
In accordance with one embodiment of the invention, a ballast for a low pressure gas discharge lamp includes an on-off indicator for a lamp that preferably has heated filaments. Three examples of ballasts for lamps with heated filaments are: (1) a program start ballast, (2) a rapid start ballast, and (3) a controlled preheat ballast.
The ballast includes a load circuit with a lamp. The ballast further includes a driver for supplying AC load current to the lamp. Such driver includes circuitry for shutting off the load current in the presence of a lamp fault condition. The ballast also includes a pair of nodes that have voltage across them when the lamp operates normally, and that have substantially no voltage across them when the lamp is off. This condition typically occurs in ballasts for powering lamps that have heated filaments.
An on-off lamp indicator circuit includes a light-emitting device and is coupled to the pair of nodes for causing the light-emitting device to emit light when the driver supplies load current to the lamp and for causing the light-emitting device to cease to emit light when the driver no longer supplies load current to the lamp.
Beneficially, the ballast can be of the mass-produced type for powering conventional fluorescent lamps, such as that of U.S. Pat. No. 6,366,032, by Allison and Moore and which is assigned to the instant assignee. The ballast described in the foregoing patent automatically shuts off power to the lamp when a lamp fault is detected. In a preferred form, the on-off indicator circuit can be powered from available nodes across which voltage of a lamp filament is present, so that the ballast does not need to be changed mechanically.
The invention may be useful in connection with a conventional fluorescent lamp, to economically provide a remote confirmation of proper operation, for instance.